1. Field of the Invention
The present invention relates to cross-linked interpolymers of ethylene and maleic anhydride. More specifically, it relates to ethylene-maleic anhydride interpolymers of improved hydrolytic and electrolytic stability which are cross-linked with a controlled amount of triallyl isocyanurate.
2. Description of the Prior Art
Numerous cross-linking agents have been considered for ethylene-maleic anhydride interpolymers. Vinyl esters have been widely used in recent years for this purpose. Another useful cross-linker is divinyl benzene.
Cross-linked interpolymers of ethylene and maleic anhydride find extensive use when dispersed in water as viscosity control agents. They act as bodying or thickening agents in the preparation of textile printing pastes, as stabilizers for detergents and as clay beneficiating agents useful in the preparation of drilling muds. Such interpolymers, generally in the form of gels, must be hydrolytically stable, however, so that their viscosity does not decrease upon standing or else they are useless in these particular applications.
Many known cross-linking agents are deficient because they result in interpolymers which are unstable products, i.e., products which have the serious disadvantage of being readily hydrolyzed in water, particularly in the presence of alkalies such as are usually present in the mixtures in which they are incorporated.
According to U.S. Pat. No. 3,073,805, such hydrolysis results in solution of the interpolymers upon standing with a steady breakdown in viscosity even to the extent that the final solution is about the equivalent of the unmodified copolymer. Examples of such ineffective cross-linking agents include, according to U.S. Pat. No. 3,073,805, vinyl allyl ether, triallyl cyanurate, triallyl isocyanurate, diallyl cyanamide, divinyl carbitol, divinyl sulfone, diallyl maleate and diallyl sebacate, among others. Thus, it was deemed surprising in U.S. Pat. No. 3,073,805 that diallyl ether as a cross-linker afforded production of interpolymers which were stable to hydrolysis.
U.S. Pat. No. 3,165,486 discloses that a number of bifunctional compounds which might be expected to be good cross-linking agents for ethylene and maleic anhydride copolymers, are not at all suitable for preparing interpolymers useful as thickeners for textile printing pastes. Several bifunctional compounds, including triallyl isocyanurate, proved ineffective according to the teachings of U.S. Pat. No. 3,165,486.
Thus, the prior art consistently taught that triallyl isocyanurate is unsuitable as a cross-linking agent for ethylene-maleic anhydride interpolymers intended for use as textile print paste thickeners.
Notwithstanding the improved results achieved with vinyl ester cross-linkers, there has remained a long-standing need for ethylene-maleic anhydride interpolymers having superior stability against electrolytic and hydrolytic degradation. It has been found, for example, that the shelf-life properties are deficient in some print paste concentrates containing vinyl ester cross-linked interpolymers. The problem is related to the rate of breakdown of the cross-linking agent, hence the resulting loss in viscosity of the ethylene-maleic anhydride interpolymer upon storage. Accordingly, the discovery of a cross-linker which affords superior stability of the interpolymer, especially in textile printing paste applications, would be an advancement in the art.
It is an object of the present invention, therefore, to provide ethylene-maleic anhydride interpolymers having superior hydrolytic and electrolytic stability.
Yet another object of the present invention is to provide cross-linked ethylene-maleic anhydride interpolymers which are superior thickeners for textile printing pastes.
Still another object of the present invention is to provide an improved method of cross-linking ethylene-maleic anhydride interpolymers.
These and other objects of this invention will be apparent from the following description.